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In the Forum: Horn-Loaded Speakers
In the Thread: Deep End DIY - Australian take one Macondo.
Post Subject: After Preamplifier ChangePosted by anthony on: 10/24/2025
An opportunity came along to trial a couple of dacs higher up the ladder of the dac which I already owned. The opportunity also extended to trial a newly released preamplifier from the same stable. I trialled the dacs in my room over several weeks and made my choice. Then I decided to listen to the preamplifier and the sound in comparison to my 10Y was.....
....horrible!!
Obviously there was a problem with how it was driving Melquiades...bass was so H.U.G.E. and totally out of proportion...an impedance issue. Got on a Whatsapp call to the designer in Cyprus and confirmed that the output impedance of the trial preamplifier was 78ohm whereas the 10Y pre is 12ohm...yep, the preamplifier output was more closely following the input impedance of the Melquiades which starts higher in the bass and finishes lower in the highs. It immediately clicked in my head that this higher output impedance may provide opportunity for higher bass output for Macondo and an overall less attenuated tuning so the next day I measured and re-jigged the Melquiades filters until they give the same frequency response with the trial preamp as with the 10Y preamp. Using alligator clips I could change filters and swap preamplifiers without turning off the amplifiers in about 2 minutes flat so I was listening apples-to-apples.
Not only was the output of the Bass Cannons much louder with this arrangement, but there was more adjustment available to go louder if I wanted it. Only foreseable problem would be that I would need more gain from the RAAL HF channel if I lessened attenuation on all the other channels. Did my listening comparisons over the weekend and it was not even close even though the measured frequency response of the two preamplifiers were much closer to identical than I would have thought possible with the filter changes.
So I want this thing, how to make it fit?
The major problem are my remaining low output sources: the 10Y has 18dB of gain and the trial pre only 3dB. That is a huge difference in potential output. The AV processor had recently been swapped to the Trinnov which has plenty of output, which meant the only low output source remaining is my phonostage. I re-measured the Melquiades inputs and 10VRMS from the preamp is about 4VRMS after the filters which is just below the onset of rising distortion in the first of the Melq channels approaching clipping...but it is also ridiculously loud. The trial pre can put out a maximum of 5VRMS which is only a bit less than 'ridiculously loud' so it's output is fine, just the gain is too low...time to see how much more bass output I can get with the higher output impedance preamp.
So I set to measuring and re-tuning the filters in Melquiades to suit the trial preamp (and no longer suit the 10Y preamp). Turns out that the Bass Cannons can be tuned 14dB louder than their original output with the 10Y preamp...and so can all the other channels apart from the RAAL HF channel which is limited to its original output SPL.
This means that with the trial pre and the new filter tuning my low output phonostage is 14dB louder through Macondo before adding any preamp gain, which means things are going to be perfectly fine. I've just skinned the cat a different way!
So much has been learnt by measuring and re-tuning Melquiades/Macondo this year that it is now obvious that the low output impedance preamplifier was in this situation a hindrance. It has not eased all of my concerns with Meql/Mac...some of those are unchanged...and I will talk about them next.
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